In injection molding of thermoplastic material it is desirable that the parts produced be uniform in shape, size, weight, strength and appearance. These characteristics are partly dependent upon the properties of the material and the design of the mold, but they are also the result of the control exercised over the molding operation. Such characteristics are dependent upon the temperature of the material, the rate of flow of the plasticized material into the mold and the density of the material in the mold.
The temperature of the plasticized material, or melt, being injected into the mold must be maintained within its working range. The rate of flow of the plasticized material into the mold determines the flow pattern in the mold, how well the mold is filled and the orientation of the molecules of the material. The rate of flow is dependent upon the viscosity of the melt and the pressure applied to it, the viscosity, in turn, being inversely related to temperature. The density of the material in the mold is dependent upon the complete filling of the mold, the viscosity of the melt in the mold and the pressure applied to it.
There is a fundamental relationship between the pressure, volume and temperature of a plasticized material expressed in the Spencer and Gilmore equation: EQU (P + .pi.) (V - .omega.) = RT
in which
P = plastic pressure PA1 V = plastic volume PA1 T = plastic temperature PA1 .pi. = constant for the plastic PA1 .omega. = constant for the plastic PA1 R = constant for the plastic
It will be seen that a change in any variable will result in a change of at least one other variable, so that manipulation of one variable may be employed to compensate for changes in another variable.
During the primary injection portion of the molding cycle, the primary injection pressure supplied to move the ram has been held substantially constant. As the ram is moved forward by the primary injection pressure the ram is moved at a substantially constant speed while the melt is injected through a nozzle into the mold cavity until the cavity is substantially filled, being limited only by the rate of flow of the melt through the gate, which rate is dependent upon melt viscosity. After the mold cavity has been initially filled, the ram continues to move forward, but at a greatly reduced speed, as the melt is compacted in the cavity, until the mold pressure plus the pressure drops in the flow passages equals the pressure exerted by the ram on the melt, at which time forward movement of the ram ceases. The mold pressure builds up slowly until the cavity is initially filled, after which it rapidly increases while the melt is being compacted until it equals the pressure exerted by the ram.
Without any control, if the viscosity of the melt had increased, as in response to a variation in composition, the opposing pressure on the ram would increase, slowing flow of the melt into the mold cavity and so reducing the time for compaction. The melt begins to set-up as soon as it enters the mold. With the slower flow the melt has a longer time to set-up before the cavity is filled and so builds up a greater resistance to compaction, the result being that the peak mold pressure is lower. Finally, as a result of the lower peak pressure, the subsequent parts will be smaller and lighter than before. If the viscosity of the melt had decreased, the cavity would fill faster, the melt would not set-up as much before the cavity was filled, the compaction time would be longer, the peak pressure would be higher, and the subsequent parts would be larger and heavier. The change in length of the finished parts is due to resilience when the pressure is removed, while the change in weight is due to different densities resulting from the difference in compaction.
Many different systems have been employed in efforts to provide the most consistent results at the lowest possible price. When manual controls are employed, the quality of the product is dependent upon the experience and skill of the operator. Automatic controls usually produce more uniform quality throughout a production run and also from run to run. Some automatic controls are so simple that they do not produce satisfactory results, while others are so complex they cannot be economically justified. Better and less expensive controls are always sought.
Some automatic controls have controlled the speed of the ram as it injects the melt into the mold cavity by adjusting the primary injection pressure. This compensates for changes in viscosity while the mold is being filled, but the adjusted pressure is also employed to compact the material in the mold. If the melt viscosity has increased, the primary pressure is adjusted upward to compensate for the resulting reduced flow. This will typically produce more dense, heavier and longer parts than without controls. Conversely, if the melt viscosity decreases, the primary pressure is adjusted downward typically producing less dense, lighter and shorter parts than without the control. Controls dependent solely upon ram speed tend to overcompensate for viscosity changes.
Other automatic controls have controlled the pressure in the filled mold cavity by adjusting the primary injection pressure, but the adjusted pressure is also employed to inject material into the mold during the following molding cycles. An increase in mold pressure at a predetermined time in the primary injection cycle indicates a decrease in melt viscosity, while a decrease in mold pressure indicates an increase in melt viscosity. If the melt viscosity has increased, the mold pressure will decrease, requiring a compensatory upward adjustment of primary injection pressure and typically resulting in production of shorter and lighter parts than without the control. Conversely, if the melt viscosity has decreased, the mold pressure will increase, requiring a downward adjustment of primary injection pressure and typically resulting in production of longer and heavier parts. Mold pressure control thus tends to under-compensate for viscosity changes.